Thick film heating elements have been long sought after because of their ability to provide versatile designs, high power densities, uniform heat and rapid heating and cooling. These types of element designs are very efficient for direct heating either by placing the thick film element in contact with the component being heated or when they are required to radiate directed heat to the surroundings.
A voltage is applied to the resistive thick film either via conductive tracks or directly to the resistive thick film. This is a desirable element design, as it is low-profile and lightweight, provides rapid heat up and cool down times, provides very uniform heat, and delivers power at low temperatures resulting in safer operation of the heating element.
Metal substrates such as aluminum and aluminum alloys and austenitic grades of stainless steel, such as 300 series stainless (300SS), are desirable for this application because of their excellent thermal performance characteristics. Aluminum and aluminum alloys are particularly desirable for this application because they have a thermal transfer 10 to 20 times that of stainless steel making thick film heaters on these substrates thermally fast acting and have a low density making for a very light, efficient heating element.
Prior art shows an insulating layer (glass enamel) applied to the substrate to electrically insulate the resistive thick film from the substrate. Glass based products produced by companies such as DuPont, Ferro and (Electro-Science Laboratories, Inc. (ESL) use a combination of melt flowable glass binder and insulative filler components. Various combinations of metal oxides in the thick film glass frit lower the melting temperature of the glass so that it flows and produces a continuous glass matrix containing the filler material at suitable firing temperatures.
Typical thick film glass frits are designed to fire at temperatures in excess of 800° C. and are typically used on substrates made of ferritic stainless steels, such as 400 series stainless (400SS). However, it is difficult to produce viable electrically insulating layers on lower temperature metal substrates such as aluminum and aluminum alloys, which have a low melting temperature (less than 660° C.) or other substrates which have a relatively high coefficient of thermal expansion (22-26 ppm/K). Enamel-based insulating layers commonly used for ferritic stainless steel substrates cannot be used for aluminum or aluminum alloy substrates or austenitic stainless steels substrates, as mismatched thermal expansion coefficients result in cracking of the electrically insulating layer during initial processing or under thermal cycling of the heater. Furthermore, these enamel coatings need to be applied at temperatures typically greater than 600° C., which is too close to the melting temperature of the aluminum or aluminum alloy substrates to produce a stable electrically insulating layer.
Melting temperatures below 600° C. can be achieved but have several limitations. Many of these insulators have lead or Cd in the thick film frits. However, the thick film formulations used to produce this element must be lead free in order to comply with the RoHS Directive adopted by Europe in 2006. In addition, these insulators do not have the required dielectric strength to meet regulatory safety standards.
Low to no melt flow polymer formulations such as polyimide may be used to form an electrically insulating layer on low temperature substrate materials. However, these polymer formulations have either (1) a low temperature limitation, (2) are not able to transfer the wide range of power densities required for consumer and industrial heating element applications into heating the substrate, (3) do not provide the required electrical insulation performance, (4) cannot withstand the resistive thick film processing conditions, or (5) compromise the integrity of the deposited resistive thick film.
The above problems with traditional insulating materials necessitate a unique materials solution for substrates having a low melting point or high coefficient of thermal expansion (CTE).